its not the strength of the epoxy/fiberglass layup but the bond of the epoxy to the piling. i assume the piling is pressure treated? not a strong bond. the rebar will be subjected to bending loads .the compression side in your drawing will be a fulcrom and the pileing will be a lever. a shock load at the top of the piling(missjudged docking, storm surge etc ) will shear the bond of the epoxy at the tension side and put bending load on the rebar. rebar is strong in many ways but bends fairly easily. three 2x6's per piling with counter sunk lag bolts is a better fix.